The possibility of hereditary susceptibility to sudden infant death (SID), suggested by anecdotal and epidemiological evidence, has not been systematically investigated. The first objective of the proposed research is to determine the relative risk of SID episodes among monozygous and dizygous twin pairs in order to assess the imputed balance between heredity and environment as determinants. The second objective is to compare the incidence of SID among parental siblings of proband families with and without multiple SID occurrences in order to assess the likelihood of polygenic determinants. Several thousand parent members of the National Foundation for Sudden Infant Death (a cosponsor) will be asked to complete a questionnaire to elicit the required data. Results should have direct application for parental counseling as well as theoretical implications for future research.